Color inkjet printers are well-known and generally incorporate multiple printheads in a scanning carriage which scans left to right and right to left across a medium while the printheads eject droplets of ink. The printheads are typically housed in one or more print cartridges either containing ink or having ink supplied to them from an external source. The ink is channeled to ink ejection chambers formed on a substrate associated with each printhead. Within each of the ink ejection chambers is an ink ejection element, such as a resistive heater or a piezoelectric element. A nozzle plate resides over each printhead such that each nozzle is aligned over an ink ejection chamber. Each printhead may have hundreds of nozzles for printing at 300 dots per inch or more. As energization signals are provided to the ink ejection elements as the printheads are scanned across the medium, ink droplets are ejected from the nozzles to create a pattern of ink dots to print text or an image. High quality printers typically print in both directions to increase throughput.
Color printers typically include a black printhead, a cyan printhead, a magenta printhead, and a yellow printhead aligned in the scanning carriage so that they scan over the same portion of a medium. Even though an image to be printed may ultimately require a full saturation primary color or a full saturation composite color, it is desirable not to print high density print swaths in a single pass. Such single pass printing is not desirable for many reasons. For example, a defective nozzle or ink ejection element would result in a white horizontal line across the medium. A single pass to deposit all the ink needed for the image may provide too much ink in too short of a time to be absorbed by the medium. This would result in excessive ink bleed, excessive drying times, and cockling (warping) of the medium. Also, a single pass may not be sufficient to provide the desired color saturation.
For at least these reasons, high quality printers use multiple passes, when appropriate, such that only a fraction of the total ink required for the image is deposited in a single pass, and any areas not covered by the first pass are filled by one or more later passes.
Since the arrangement of the color printheads in the scanning carriage is fixed (such as the order: black, cyan, magenta, and yellow as viewed from the front of the printer) and all three primary colors (and possibly black) will generally be printed during each pass for a full color image, yellow ink will always be deposited first going from left to right, and cyan ink will always be deposited first going from right to left. Even though a particular composite color requires a particular ratio of, for example, yellow and cyan, this composite color will appear differently depending upon whether cyan ink overlaps yellow ink or whether yellow ink overlaps cyan ink.
When printing using a multi-pass print mode in both directions, color shift results from two or more overlapping colors of ink being deposited on the medium in different orders. A color shift is any detectable difference in color, including becoming lighter or darker. Because the medium is typically stepped after each pass, color shift is generally apparent by horizontal bands which alternate from lighter to darker (or from one color to another color) due to the colors overlapping in different orders during sequential passes. Prior applications have used large numbers of passes to offset this color shift effect. For example, four or eight total passes, each pass depositing an equal amount of ink, causes the error to be dispersed and largely unnoticed. Multiple pass techniques in an inkjet printer have been described in the following U.S. patents assigned to the present assignee and incorporated herein by reference: U.S. Pat. Nos. 5,555,006; 5,376,958; 5,276,467; and 4,965,593. These patents are also cited for their description of masking particular nozzles during particular passes, referred to as print masking, and to illustrate that such print masking techniques are well-known and need not be described herein in detail.
Another defect from multiple passes in both directions results from bi-directional alignment errors of the carriage whereby dots in one pass intending to overlap dots from a previous pass do not precisely overlap those dots.
What is needed, is a multi-pass technique that mitigates the above-described drawbacks.